She knew.
Twilight knew. Her student knew that all of this was happening. And nopony listened. Least of all her.
Alright, Twilight didn't know exactly what was happening. But she suspected something was wrong with Cadance. She did have her cornered, and, for just a moment, Cadance looked afraid. Not afraid of Twilight, but afraid that she'd been found out.
Celestia should have seen it then. She should have noticed that her niece's magic was off. Should have seen that she was unusually cruel and short with everypony. Certainly, brides had been known to be short-tempered before. But…she would never have suspected it with Cadance. Couldn't have. Cadance was always so good and kind under any kind of pressure (and being a royal of any degree brought its fair amount of pressure).
Twilight knew that. She had known Cadance since she was a filly, after all. She knew something was wrong and nopony listened. They just told her off and then left her to feel guilty. Went to go focus on wedding plans while her most faithful student almost single-handedly unfolded an invasion plot that was happening right under her nose.
Sometimes, Celestia wondered how she could be such an idiot.
And now here she was, trapped in some slimy green cocoon, for lack of a better description, hoping against hope that her subjects could retrieve the Elements of Harmony. Which, she realized too late, were locked in a chamber that only she could open.
Really. How did such an idiot come into power? She was really beginning to question her subjects' wisdom for having so much faith in her.